It is generally well known in the art that irradiation, such as by electron beam irradiation, of certain polymeric film materials results in the irradiative cross-linking of the polymeric molecular chains contained therein and that such action generally results in a material having improved heat shrink properties, abuse resistance, structural integrity, tensile strength, puncture resistance, and/or delamination resistance. Such physical improvements from irradiation, in particular the improved heat shrink properties, are discussed in U.S. Pat. No. 3,022,543 (1962) to Baird et al. Many of the other physical improvements also are discussed at columns 2 and 8 of U.S. Pat. No. 4,178,401 (1979) to Weinberg and at column 4 of U.S. Pat. No. 3,741,253 to Brax. Furthermore, it is also known from U.S. Pat. No. 4,525,257 (1985) to Kurtz et al that low level irradiation under 2 MR of narrow molecular weight distribution, linear low density ethylene/alphaolefin (LLDPE) particulate copolymer sufficient to introduce cross-links into the particulate copolymer but insufficient to provide for significant measurable gelation produces improved copolymer rheology providing increased extensional viscosity during film fabrication, i.e. the bubble is more stable as the LLDPE is more easily stretchable.
That cross-linking of polymers may be accomplished chemically through utilization of chemical cross-linking agents is also well known to those of skill in the art. For instance, cross-linking agents, such as allyl compounds or organic peroxides, have been used to cross-link polyethylene polymers and copolymers. A general discussion of chemical crosslinking can be found at pages 331 to 414 of volume 4 of the Encyclopedia of Polymer Science and Technology, Plastics, Resins, Rubbers, Fibers published by John Wiley & Sons, Inc. and copyrighted in 1966. This document has a U.S. Library of Congress Catalog Card Number of 64-22188 and the referenced pages are hereby incorporated by reference. Typically, the chemical cross-linking agents react with the polymer to form a solid or highly viscous cross-linked copolymer. Furthermore, it is also known from U.S. Pat. No. 4,515,745 (1985) to Churma et al assignors to Union Carbide and U.S. Pat. No. 4,675,364 Churma et al assignors to Viskase that when an organic peroxide chemical cross-linking agent is used in manufacturing an extruded film having at least one layer of EVA, in a controlled amount small enough (0.05 to 0.015% agent by weight of the EVA layer) not to form gel measurable by ASTM test method number D 2765 (ASTM D2765 is further discussed below), i.e. no measurable gel means the EVA containing a crosslinking agent is soluble in a solvent such as xylene, then the rheology during extrusion is modified, i.e. the bubble is more stable as the EVA is more easily stretchable. A drawback of organic peroxides is the difficulty in controlling the amount used. Organic peroxides are free radical generators and will react as soon as they come in contact with the heat of the extruder. Thus, they can gum up the workings of the extruder before the polymeric film has a chance to exit the extruder. U.S. Pat. No. 4,614,764 (1986) to Colombo et al shows greater bubble stability in the blown tubular film extrusion process by controlling free radical generator type of cross-linking agents such as organic peroxides, by first modifying LLDPE in the molten state with a free radical generator and then blending the resultant with LLDPE, so the reaction has already occurred prior to blending this modified polymer with LLDPE in the extruder during blowing a film.
Also of interest is that electron beam irradiative cross-linking of the polyethylene insulation of high voltage power cables has a foaming problem which is alleviated by use of a chemical cross-linking agent, such as ethylene glycol dimethacrylate, diallyl maleate, dipropargyl maleate, dipropargyl monoallyl cyanurate, or triallyl cyanurate, which agent accelerates the electron beam so low megarad dosage of irradiation can be employed. This is discussed in "Development of Radiation Cross-linking Process for High Voltage Power Cable", Sasaki et al, Japan Atomic Research Institute, Takasaki, Japan, (1979) Vol. 14 Radiation Physical Chemistry, pages 821-830, and "Suppression of Discharge Breakdown of Polyethylene Insulation During Electron Beam Irradiation to Power Cable", Sasaki et al, Takasaki Radiation Chemistry Research Establishment, JAERI, Takasaki, Japan, (1981) Vol. 18, No. 5-6, Radiation Physical Chemistry, pages 847-852. Of related interest is U.S. Pat. No. 4,576,993 (1986) Tamplin et al assignors to Raychem which discloses LDPE cross-linked by 0.2 to 5 weight percent of pro-rad such as triallyl cyanurate and 2 to 80 megarads of irradiation for use in dimensionally recoverable articles such as high voltage insulation.
Also of interest is U.S. Pat. No. 4,374,223 issued Feb. 15, 1983 to Raamsdonk et al assignors to Olympia Werke which shows an elastic covering for closing off a nozzle outlet area containing at least one nozzle orifice of an ink printing head filled with an aqueous liquid, said covering comprising as elastomeric material having viscoelastic properties for filling exceedingly small cavities and interstices in the area to be closed off, comprising by weight: 100 parts ethylene-vinyl acetate copolymer having an ethylene: vinyl acetate ratio of 40:60; 20-40 parts talc, as mineral filler; 0.5-15 parts polycarbodimide hydrolysis protection agent; 0.2-1 parts age retardant (antioxidant); 0.5-2 parts triallyl cyanurate; 3-5 parts 1-1-Bis(tert-butyl peroxy)-3,3,5-trimethyl cyclohexane; and 2-10 parts polyethylene glycol of molecular weight about 200.
However, it has been unexpectedly discovered that the combination of a pro-rad type of chemical cross-linking agent with irradiation has not only obviated the problem of carefully controlling the amount of chemical cross-linker so as to avoid gumming up the extruder but also has resulted in the orientation process for manufacturing polymeric films having a decreased time, i.e. increased speed or rate.